For a given hash function H, to prevent substitution of a message M1 for a message M2, it is
necessary that:

A.
H(M1) = H(M2)

B.
H(M1) <> H(M2)

C.
H(M1) < H(M2)

D.
H(M1) > H(M2)

Explanation:
The correct answer is H(M1) <> H(M2). If H(M1) = H(M2) then the message digests of both M1 and
M2 would be equal, and one would not know that one message was substituted for the other. The
other answers are distracters.